Obtaining containers through blowing special preforms suitably heated within a mould of the desired shape is a technique widely used in the packaging industry, in particular for manufacturing beverage bottles.
Among the various useable techniques, stretch blowing consists in a simultaneous action of mechanical stretch performed by a spindle which moves longitudinally within the preform and blowing by introducing air at high pressure. This technique has several advantages, including improvement of resistance against traction, barrier properties and transparency of the container, as well as the possibility of obtaining containers starting from preforms of lighter weight, with ensuing advantages both from an economic and environmental point of view.
Stretch blowing provides for a pre-blowing, simultaneous to the mechanical stretch action, at a first pressure, and a final blowing at a second pressure.
Known stretch blow moulding machines provide for a double pneumatic system. The first pneumatic system provides air for pre-blowing at constant pressure, settable at values between 2 and 10 bars; the second pneumatic system instead provides blowing air for forming the final bottle and it also operates at constant pressure values, settable between 20 and 40 bars.
This solution is extremely limiting and complex, in that the management of two independent plants complicates the provision of the machine.
Another drawback of the conventional machines lies in the fact that notch pressurization profiles are used, with two different pressures, each at a constant level. Considering the fact that the final mechanical characteristics of a bottle depend on the thermo-forming strategies thereof, having notch deformation profiles could limit the possibility of optimising the bi-orientation phenomenon of the final material, with the ensuing effect that the crystallisation of the material is not optimised.
Last but not least problem lies in the management of the electromechanical inertia of the switching solenoid valves, which offer variability in the opening and closure delay, in turn a function of the variation of performance between one valve and the other; the deterioration of this uncertainty is accentuated by the electro-actuation response times with very wide (30-40 ms) and variable time windows, with the possibility of partly jeopardising the process repeatability.